The present invention relates to a method of inhibiting corrosion of carbon steel piping of condensate and feed water systems in power generating plant at shut-down period thereof.
In a system in which water dissolving oxygen and carbon steels are in contact, carbon steels are corroded, and in stationary water, the higher the concentration of dissolved oxygen, the greater the degree of corrosion. Particularly, when water dissolving oxygen of about 40 to 30,000 ppm, carbon steels in contact with water are subjected to corrosion, for which inhibitive measures are required.
In boiling water reactor (BWR) plants, particularly during the shutdown period due to the periodical inspection and others, the piping of the condensate and feed water systems are exposed to water of dissolved oxygen concentrations of as high as 5 to 8 ppm under the condition open to the air, so that carbon steels for the piping are corroded. Corrosion products (mainly composed or iron oxides and called "crud") might be brought into the reactor at the plant starting time and deposited on fuel rods, thereby causing the decrease in thermal efficiency and the failure of the fuel rods. Furthermore, crud deposited on the fuel rods, after activated, is in danger of peeling off and being redeposited on components, such as the piping of a reactor recirculating system, thereby causing the increase in the surface dose rates of components such as the piping and the increase in radiation exposure doses for persons engaged in the periodical inspection.
From these reasons, corrosion inhibitive measures are regarded as an important subject with respect to the piping of the condensate and feed water systems particularly during the shutdown period of BWR plants.
A drain-drying method, as referred to "hot drain off", has been so far applied to some plants for inhibiting corrosion in a system in which water dissolving oxygen and carbon steels are in contact, particularly for the piping during the shutdown period of BWR plants. In this method, the piping are drained after the plane has been shutdown, but before feed water gets cold, thereby having their surfaces dried by remaining heat. However, such a method can not be necessarily applied to any plant because of the structural differences among plants, and also a great amount of liquid radioactive waste resulting from the drainage provides a problem in its treatment. Furthermore, this drain-drying method is not suitable in cases where the shutdown period is short because of its complicated operation.
Besides, U.S. Pat. No. 3,663,725 discloses method for inhibiting corrosion of piping in water cooled reactors by introducing into feed water a small amount of oxygen and hydrogen (about 100 to 300 ppb oxygen and a stoichiometric quantity of hydrogen).
As other method of inhibiting corrosion, in fossil fuel power plants, when a shutdown period is short, a wet layup method based on the addition of hydrazine is applied, while in the case of a long shutdown period, a dry layup method in which pipes are filled with nitrogen gas after drainage is applied. These method, however, can not be applied to inhibit corrosion in BWR plants because there are problems of post-treatment such that hydrazine must be removed before the following starting time or nitrogen gas must be deaerated.